Subject: 8f.10   Cracking/Breaking Cranks
From:  Jobst Brandt <jbrandt@sonic.net>
Date:    Thu, 14 May 2009 13:03:19 -0700

Major brand aluminum cranks break because they have high stress at
various locations and because aluminum alloys have no distinct fatigue
threshold.  The worst locations are at the pedal eye and where the
spider fingers join the right crank.  The pedal eye is a high stress
zone because the joint is incorrectly designed, but since it is a
standard, it may not be changed since it seems to work.

This interface violates a basic design rule that loads should not be
supported radially on threads.  Threads should be used to pull
elements together.  Because the pedal joint has no radial press fit,
it always moves and causes fretting erosion while generating radial
cracks.  These cracks grow in depth and cause the pedal eye of the
crank to break.  A better joint, similar to an automobile lug nut,
would have tapered face instead of bearing flat against the crank.
This would furnish the preload and pressure angle required by this
interface (it has been prototyped and works well).

Presence of a left hand thread is proof that this is a faulty design
because the fretting works to unscrew the thread that should not be
moving.  Screw threads are fasteners, not load supporters.  Wherever a
left hand thread appears, it is a faulty design unless it is a
turnbuckle that demands left/right threads that are axially loaded.

The thin web between the spider and crank, another common failure
point on cranks, like the Campagnolo Record or Shimano Dura Ace, a
place that has often been filed away to male a radius.

Aluminum has no safe fatigue limit but is progressively more durable
peak as stresses are reduced.  In contrast, steel has a threshold
stress below which failures cease.

Cranks loosening on BB Spindles can be visualized by seeing the crank
as being made of Rubbermaid(R) household plastic.  The crank, once
properly installed and the retaining bolt in place, squirms on the
square taper under torque.  During these deformations the crank can
move only in one direction because the bolt prevents it from coming
off.  The crank "elbows" its way farther up the square taper.  Proof
that the crank squirms is shown by the fretting rouge found on the
spindle, whether lubricated or not, when a crank is removed after
substantial use.

As was mentioned by various observers, after use left crank bolts lose
more preload than right ones which could be anticipated because the
cranks are loaded differently.  Loss of more preload does not mean the
left crank is looser.  Actually it is tighter, only the bolt is
looser.  The left crank is more heavily loaded because it experiences
offset twist from the pedal at the same time it transmits torque to
the spindle.  The right crank, being connected to the chain,
experiences either spindle torque from the left pedal or twist from
the right pedal but not torque and twist at the same time.

In this squirming mode, cranks wander away from the retaining bolt and
leave it looser after the first hard workout (for riders of more than
150 lbs).  The bolts should NOT be re-tightened, assuming they were
correctly tight when installed.  Cranks have been split in half from
repeated follow-up tightening, especially left cranks.  The spindle
should be lubricated before installing cranks.  A wipe of a mechanic's
finger is adequate since this is to prevent galling the interface
while advancing the press fit.  To prevent losing a loose crank bolt,
the "dust" cover that acts as a lock nut (Campagnolo type) should be
used.

When a BB crank spindle breaks, the greater stress on the left side
causes this end to breaks from fatigue.  A fatigue crack generally has
a crystalline appearance and usually takes enough time to develop that
the face of the fracture oxidizes so that only the final break appears
clean.  Because a (crack) notch concentrates stress, the crack
accelerates its advance once it has stated.

Instructions to not lubricate spindles before installing cranks are
common but this admonishment is not found it in manufacturer's
instructions.  Standard machine practice is to lubricate tapers with
no ill effects.